1. Technical Field
The invention includes embodiments that relate to an electrochemical cell. The invention includes embodiments that relate to a high-temperature rechargeable electrochemical cell. The invention includes embodiments that relate to a method for detecting a state-of-charge of the high-temperature rechargeable electrochemical cell.
2. Discussion of Related Art
There have been numerous attempts in the past to provide state-of-charge (SoC) or battery condition indicators, which allow users to readily determine the SoC of storage batteries, particularly those used in automobiles. These attempts have ranged from the extremely simple to very sophisticated. Commercial battery products that track SoC typically use a shunt to measure the current being passed and integrate this current over time to track the SoC of connected cells. While over short periods of time these measurements may be quite accurate but over very long periods of time there can be a drift due to the finite error of the electrical components such as the shunt, voltage recording, and integration of the signal over time. Further these measurements are made on the assumption that all cells have received an amount of current and that all the current received is used for the expected reactions in the cell, in other words these measurements assume that the cells exhibit 100 percent coulombic efficiency. However, this is not true for most batteries. The other issue with all other batteries is self discharge which is uncontrolled and different for each cell which makes the use of the current counting type SoC indicator unreliable for cell to cell variation.
Tracking the SoC is usually included in a battery management interface (BMI). The BMI functions as an onboard safety device, limiting the minimum and maximum voltages, temperatures, and SoC that the battery is subject to throughout its life. The BMI also may include a counter circuit for counting the number of charge/discharge cycles producing a count representative of the amount of energy dissipated and therefore indirectly of the amount of energy remaining in the battery system. However while BMI can track the SoC of electrochemical cells, the circuitry may have some finite error. This SoC sensor may not provide a means for tracking capability over long periods of time, which will allow much more accurate control of the SoC of the cells being used. If the SoC is not accurately controlled the cell life may be reduced, as over-charging or over-discharging can lead to premature cell failure. Even if the error is very small, over long periods of time the error can become significant, and the actual SoC of the cells may become very different from the recorded SoC. Further the BMI may not provide continuous internal means of determining capacity and may not be capable of providing an indication once full battery capacity is restored.
It is well documented and accepted that the available energy in a battery is a function of the conditions to which the battery has been subjected. Capacity remaining is a complex function of current drain, temperature and time. Therefore there remains a need, however, for a low cost, reliable, accurate, continuous, automatic-reset SoC indicator which is both easy to use and easy to manufacture.